Method of forming an electric capacitor



March 12, 1963 1.. R. KOLLER METHOD OF FORMING AN ELECTRIC CAPACITOROriginal Filed May 15, 1957 lnvenfor: Lew/s R. Ko/ler x pis Ah om e y.

United States Patent Ofiiice fidllfll Patented Mar. 12, 19553 3,681,201METHQD F FQRMHNG AN ELECTRKC GAPACHQR Lewis K. Keller, Schenectady, NFL,assignor to General Electric Company, a corporation of New Yarn Originalapplication May 15, E57, $91. No. assess,

now Patent No. 2,975,345, dated Mar. 14, 1.961. D:- vided and thisapplication May 2%, H60, Ser. No.

3 Claims. (ill. 111-215) The present invention relates to improvedelectric capacitors and to methods for manufacture thereof.

This is a division of my copending application Serial No. 659,285, filedMay 15, 1957, now Patent No. 2,975,- 345, and entitled ElectricCapacitor.

It is well-known that electric capacitors may be manufactured by vacuumevaporation techniques wherein either the capacitor dielectric, theelectrodes thereof, or both, are formed by evaporation and condensationof the constituent materials in an evacuated enclosure. Capacitorsmanufactured in this manner exhibit high capacity per unit volume.Additionally, due to the accurate controls which may be maintained inthe evaporation process, capacitance values may be accuratelycontrolled. Such capacitors are of particular utility for use withprinted circuits.

The process of manufacturing evaporated capacitors, however, is notwithout its diificulties. Materials posses sing high dielectricconstant, high dielectric strength, low power factors and othercharacteristics, desirable in dielectric materials, are often not wellsuited to vacuum evaporation techniques for the production of thinlayers thereof. Thus, for example, quartz and aluminum oxide, twocommonly used dielectric materials, are extremely dilficult to evaporatewith proper control.

Accordingly, one object of the present invention is to provideevaporated electric capacitors having high capacity, high dielectricstrength, and low power factor.

Another object of the present invention is to provide electriccapacitors, the dielectric of which comprises a material having gooddielectric characteristics which may be readily formed into thin layersby vacuum evaporation.

In accord with one embodiment of the invention, I provide an electriccapacitor comprising an amorphous, vacuum evaporated layer of zincsulfide containing a small amount of manganese spaced between a pair ofconducting electrodes. The amorphous layer of zinc sulfide and manganesepossesses highly desirable dielectric characteristics and may readily beprepared in thin, uniform layers by vacuum evaporation. In accord withanother feature of the invention, a rolled cylindrical capacitor isformed by simultaneously evaporating a dielectric layer and a conductinglayer upon a conducting sheet as it is wound upon a suitable mandrel.

The novel features believed characteristic of the invention are setforth in the appended claims. The invention itself, together withfurther objects and advantages thereof may best be understood withreference to the following description taken in connection with theattached drawing in which;

FIGURE 1 represents a parallel plate capacitor constructed in accordwith one embodiment of the invention;

FIGURE 2 represents apparatus suitable for preparing the device inFIGURE 1;

FIGURE 3 represents an alternative device constructed in accord with theinvention; and

FIGURE 4 represents apparatus suitable for the pro duction of the deviceof FIGURE 3.

V In FIGURE 1, a parallel plate capacitor, represented generally as 1,comprises a dielectric layer 2 in paralle spaced relation between a pairof conducting electrodes 3 and 4. Dielectric material 2 comprises anamorphous layer of high purity zinc sulfide containing from 0.01 to 10.0weight percent of elemental manganese. Capacitor plate or electrode 3 iscomposed of a conducting material and may conveniently be a thinmetallic plate of a suitable material as for example silver, aluminum,copper, tin or any other metal conventionally utilized as a capacitorplate. Since capacitor 1 is prepared by vacuum evaporation, it isgenerally preferable that plate 3 be a rigid plate upon which layers 2and 4 may be evaporated. it is not necessary, however, that capacitorplate 3 be a metal. Thus, for example, capacitor plate 3 may be a thinplate of glass, Pyrex glass, Vycor glass or quartz having thereupon athin conducting layer of evaporated metal, tin oxide known as conductingglass, or a reduced film of titanium dioxide.

Capacitor plate 4 may be any conducting material which may be suitablefor a capacitor plate and may conveniently comprise any of the materialsset forth with respect to capacitor plate 3. Preferably, however,capacitor plate or conducting electrode 4 comprises a thin evaporatedlayer of a readily volatiiizable metal such as silver, copper, tin,aluminum, zinc, gold or manganese.

The parallel plate capacitor illustrated in FIGURE 1 may readily beprepared in the apparatus illustrated schematically in FIGURE 2. InFIGURE 2, the apparatus comprises an evacuable reaction chamber or helljar 6, mounted upon, and vacuum sealed to, a suitable insulating basemember 7. One plate of the capacitor to be formed in the apparatus ofFIGURE 2, in this case capacitor plate 3, is mounted horizontally uponsupporting members 8 within the reaction chamber. A pair of evaporationvessels or boats 9 and 19 are mounted directly under capacitor plate 3and are symmetrically located with respect to the center thereof.Evaporation boats 9 and it} are supported by conducting support members11 and 12 respectively, which also serve as electrical contacts thereto.Evaporation boats 9 and 10 are constructed of a high resistance,refractory, conductive material such as tungsten, molybdenum or likemetals, which may be heated to incandescence by the passage of anelectric current therethrough. An exhaust conduit 13 passes through vase'7 and is connected to an exhaust pump, not shown, to maintain properlow pressure atmosphere within reaction vessel 6. A source of electricpower, represented conventionally by battery 14, is utilized to supplyelectric power to evaporation boats 9 and 1t) through potentiometers l5and 16 respectively. Potentiometers i5 and 16 may be utilized to supplya regulated electric current simultaneously or sequentially toevaporation boats 9 and 10. A resistance heating element 17, heated by asource of electricity, not shown, is located in close proximity to plate3 for de-gassing and heating purposes. Evaporation boats 9 and iii mayconveniently be located at a distance of approximately 4 to 6 inchesbelow capacitor plate 3. If only a portion of plate 3 is to have acapacitor formed thereon, as in printed circuit techniques, a suitablemask may be interposed between plate 3 and the evaporation boats.

In forming the device of FIGURE 1, in accord with one aspect of theinvention, a metallic capacitor plate, as for example a one-half inchsquare plate of aluminum is placed upon support members 8. A smallquantity of high purity zinc sulfide mixed with suflicient elementalmanganese to cause a film formed upon plate 3 to contain 0.01 to 10.0weight percent of manganese is placed within evaporation boat 9. it hasbeen observed that the proportion of manganese within the evaporatedfilms is usually greater than the proportion Within the evaporationboat. The exact change in proportion depends upon the system geometryand the operating conditions. A

small quantity of a material which is to comprise evaporated capacitorplate 4, as for example aluminum, is placed within evaporation boat 10.Bell jar 6 is sealed to base 7 and the apparatus if exhausted to a verylow pressure which should be no greater than approximately microns ofmercury. I prefer, however, that the process be conducted at pressuresof less than 0.05 micron. When the suitable chosen operating pressurehas been attained, electric current is supplied through evaporation boat9 containing the zinc sulfide and manganese, raising the temperature ofthe evaporation boat to a temperature in excess of 1000 C. Theevaporation of the zinc sulfide and manganese may be conducted at atemperature from '1000" C. to 2000 C. Below 1000 C., the zinc sulfideand the manganese do not. evaporate to any appreciable extent. Above2000 C, the zinc sulfide and the manganese are vol atilized too rapidly,causing the formation of an uneven particulate film upon capacitor plate3. I have found that superior dielectric layers are produced when thetemperature of evaporation boat is maintained at approximately 1500" C.

During the evaporation of the zinc sulfide and the manganese, I havefound that it is essential that capacitor plate 3, which comprises theevaporation substrate, be maintained unheated or at a temperature ofapproximately 25 C., which is approximately room temperature. This isbecause I have found that evaporated films of zinc sulfide formed atsubstrate temperatures of approximately 100 C. or higher possess acrystalline structure Whereas films of zinc sulfide deposited atsubstrate temperatures lower than approximately 100 C. are amorphous.The crystalline structure is not suitable for electric capacitors, sincethe crystalline zinc sulfide has a' lower dielectric strength, andresults in inferior dielectric characteristics in general. Accordingly,in the practice of my invention, the substrate 3, upon which the zincsulfide and manganese film is formed, is preferably maintained atapproximatelyroom temperature so that the zinc sulfide and manganesefilm is amorphous. The amorphous zinc sulfide and manganese film hasbeen found to possess highly satisfactory dielectric characteristics.However, since crystalline zinc sulfide films adhere to metal and glasssubstrates better than amorphous films, it may be desirable to firstform upon the substrate, a thin film of crystalline zinc sulfide ofinfinitesimal thickness to insure proper adhesion.

The evaporation of the zinc sulfide and the manganese from evaporationboat 9 may be carried on for any period of time sufficient to produce adesired thickness of dielectric film 2. This thickness may convenientlybe 1 to 20 microns. With evaporation boat 9 maintained at a temperatureof approximately 1500 C., and approximately 4 /2 below plate 3, thedielectric film of zinc sulfide and manganese is deposited at a rate ofapproximately 1 micron per minute. Accordingly, the evaporation time mayreadily be controlled to secure a film of zinc sulfide and manganesewhich is any desired thickness.

After the desired thickness of the dielectric layer has been secured byvacuum evaporation, the electrical power supply to evaporation boat 9 isinterrupted at rheostat 15. Electrical power is then supplied toevaporation boat 10 through rheostat 16 to cause the evaporation, upondielectric layer 2, of a thin film of a conductive material such ascopper, aluminum, silver, gold, zinc, tin or manganese, to form thesecond capacitor plate or electrode 4. The temperature at which boat 10is maintained for the evaporation of electrode 4 will, of course, varydepending upon the material which is utilized. Techniques for theevaporation of metals are well-known to the art and will not bediscussed herein. It is only necessary that the evaporated layer ofconducting material be sufficiently thick to provide good electricalconductivity. Conveniently, electrode 4 maybe approximately 0.1 to 1.0micron thick.

One evaporated capacitor prepared in accord with the present inventionwas prepared as follows:

A one-half inch square Pyrex glass plate thick was washed and cleanedwith aluminum oxide abrasive. The plate was placed in the bell jarillustrated in FIGURE 2 of the drawing. 3 grams of zinc sulfidecontaining 1.0 weight percent of manganese was placed in boat 9 and 200mg. of aluminum was placed in boat 10. Bell jar 6 was evacuated to anatmosphere of less than 1 micron of mercury. The plate was heated to atemperature of 400 C. for 5 minutes, then cooled to C. for 10' minutes.While the plate was maintained at this temperature, sufficient aluminumwas evaporated onto glass plate 3 from tungsten boat 10 to form anopaque film (a thickness of 0.1 micron). Tungsten boat 10 was heated to1500 C. in the performance of this operation. The temperature of thesubstrate was then raised to 400 C. by resistance heater l7, and thetemperature of boat 9 was raised to a temperature of 1500" C. untilavery thin film, approximately 0.1 micron thick, was deposited upon thealuminum. The thickness of this film was noted by one order ofinterference color changes of the glass plate. Thereafter, the heat wasremoved and the glass plate was allowed to cool to room temperature.After the glass plate had reached room temperature, the temperature ofevaporation boat 9 was again raised to 1500' C. until all the zincsulfide had evaporated therefrom. After the zinc sulfide had beenexhausted, evaporation boat 10 was again heated to 1500 C. and anotheropaque film of aluminum approximately 0.1 micron thick was evaporatedonto the evaporated zinc sulfide and manganese layer. The electricalpower was then disconnected, the bell jar was openedand the capacitorwas removed.

The capacitor evaporated by the above process was subjected to standardelectrical bridge capacitance measurements, and exhibited a capacitanceof 0.002 microfarad at 50 volts A.C. This capacitor was tested atfrequencies of 60, 120,180, 240, 300, 360, 600 and 1000 cycles persecond and in all instances exhibited a dielectric constant of 12 and apower factor of 0.003. With 50 volts between the capacitor plates, thecapacitor maintained these values of dielectric constant and powerfactor at temperatures of 26 C., 57 C. and 82 C. After the temperaturehad been raised to 88 C. the power factor rose to 0.023. The temperaturewas not increased to breakdown. This capacitor could not be tested fordielectric strength since the above heat test was destructive. V v

A second capacitor was made by the same steps and controls as above. A2" diameter Pyrex disk was the substrate. A 1.0" square aluminumelectrode 0.1 micron thick was evaporated upon the glass through a mask.A first layer of zinc sulfide and manganese 0.1 micron thick wasevaporated while the substrate was at 300 C. With the substrate at 25 C.a 4.8 micron thick layer of zinc sulfide and 1.0% manganese was formedand then an aluminum electrode 2 cm. square and 0.1 micron thick wasformed upon the dielectric layer. The capacitor was then tested to highvoltage breakdown, failing at a voltage of 200 v. This indicated adielectric strength of 415 kv. per cm.

The capacitor described hereinbefore is a parallel plate capacitor. Theinvention, however, it not limited to parallel plate capacitors, butincludes many other configurations. Thus, for example, capacitors inaccord with this invention may be prepared in the conventional wound orrolled configuration, illustrated in FIGURE 3, wherein alternate layersof dielectric and conducting material are wound upon a mandrel to form acylindrical capacitor.

In accord with another feature of my invention the apparatus of FIGURE 4may be utilized for the production of a wound cylindrical evaporatedcapacitor. In FIGURE 4, a bell jar l8 isrno-un-ted upon, and vacuumsealed to, base 19. Within bell jar 18, a roll 20 of a flexibleconducting material, as for example, aluminum foil, is mounted upon amandrel 21 adapted for easy rotation. Mandrel 21 is supported within jar18 by a suitable means (not shown). A thin sheet of aluminum foil 22,for example, is unrolled from roll 20 and is rolled upon capacitorassembly 23 which is wound upon a mandrel 24 and rotates therewith. Inorder to prevent the capacitor formed in accord with this feature of theinvention from being electrically short-circuited by the Winding of thecapacitor, the metallic film 22, before winding upon roll 20, issuitably treated, as for example by surface oxidation, so as to haveupon the undersurface thereof a non-conducting electrically insulatingfilm 22a. Alternatively, roll 21 may comprise a double roll of aconducting metal such as aluminum and an insulating dielectric, as forexample insulating paper. In yet another alternative, roll 20 maycomprise a thin rolled strip of a suitable organic plastic such as Mylaror polystyrene approximately 10 microns thick, having thereupon a filmof evaporated metal such as silver, aluminum or gold approximately 0.1to 1 micron thick. In all instances, however, insulating layer 22ashould be as thick or thicker than the dielectric layer to be depositedto prevent undesired changes in capacitance.

Means for simultaneously evaporating a suitable dielectric material anda suitable conducting material thereupon to form a second plate of thecapacitor are provided in the form of evaporation boats 25 and 26mounted respectively upon supporting and conducting means 27 and 28. Ashield member 29 is located between evaporation boat 25 and capacitor 23so as to direct the vaporized material evaporated therein to a firstportion 30 of the external cylindrical surface of capacitor 23. A secondshield member 31 is located between evaporation boat 26 and capacitor 23to direct the vapors of the materials evaporated therein to a secondportion 32 of the external cylindrical surface of capacitor 23. Sincethe diameter of capacitor 23 changes constantly, shields 2 9 and 3-1 maybe progressively enlarged by a suitable mechanical control means, toproperly direct the respective vapors. Both first portion as and secondportion 32 of the external cylindrical surface of capacitor 23 arechosen to be mutually exclusive of each other and to be upon thatportion of the cylindrical surface of the capacitor which has alreadybeen wound. This is because the evaporated dielectric film is generallybrittle and non-flexible. Accordingly, if the dielectric film were to bevaporized and deposited upon a portion of the metallic sheet 22, beforeit has become conformed to the cylindrical surface of the capacitor, anyfurther flexing of the film would cause flaking of the dielectric. Inpractice, foil 22 continually unrolls from roll 20 and rolls uponcapacitor 23. As a certain portion thereof passes through region 30,which is near in the direction of rotation of the cylinder, to the pointat which sheet 22 conforms to the surface of the cylinder, a suitabledielectric, for example zinc sulfide, is evaporated thereuponcontinuously. As it passes through region 32, which is remote, in thedirection of rotation, from the point at which sheet 22 conforms to thesurface of the cylinder, a suitable conducting material, for examplealuminum, is continuously vaporized over the dielectric layer. Thus, thevapors of the dielectric material are selectively directed to a firstportion of the cylinder following the point at which the sheet 22 firstconforms to the cylindrical shape thereof to deposit a thin dielectriclayer on the outer surface of the sheet. At the same time the vapors ofthe conducting material are selectively directed to a second portion ofthe cylinder angularly d-isplaced in the direction of rotation of thebody from the first portion so that a thin electrically conducting layeris deposited upon the dielectric layer. Accordingly, a continuous rollof a four layer laminate is continuously wound upon mandrel 24. Thiscontinuous roll comprises the original metallic foil 22, insulating filmor layer 22a,

6 the dielectric evaporated thereupon at 30, and the metal evaporatedthereupon at 32.

After the desired diameter of capacitor 23 has been attained, electricalpower to the evaporation boats (not shown, but identical with the powersupplies illustrated in FIGURE 2) is discontinued and the capacitor isremoved from the apparatus. Electrical contacts may conveniently then bemade to the last formed portion of the film which may then be suitablywound and glued as is conventional with this type capacitor. Althoughthe practice of this embodiment of the invention preferably is performedutilizing the zinc sulfide and manganese dielectric describedhereinbefore, other dielectrics such as MgF CaF and cryolite may also beutilized. Also, any suitable metal foil may be utilized, and theevaporated metal may likewise be any suitable volatizable metal, such asthose mentioned hereinbefore.

While the invention has been described with respect to particularembodiments in the foregoing disclosure, many modifications and changeswill immediately occur to those skilled in the art without departingfrom the invention. Accordingly, I intend by the appended claims tocover all such modifications and changes as fall within the true spiritand scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The method of forming an electric capacitor which comprises:continuously rolling a flexible laminate comprising a metallic layer andan insulating layer upon a rotatable driven mandrel within an evacuatedenclosure to form a cylindrical body having a metallic outer surface;evaporating a dielectric material consisting essentially of amorphouszinc sulfide containing 0.01 to 10.0 Weight percent of manganese andselectively directing the vapors thereof to a first portion of saidcylinder following the point at which the laminate first conforms to thecylindrical shape of the body to deposit thereupon a thin layer of thedielectric; and simultaneously evaporating an easily volatizable metaland selectively directing the vapors thereof to a second portion of saidcylinder angularly displaced in the direction of rotation of the bodyfrom said first portion to deposit a thin metallic layer upon saiddielectric layer.

2. The method of forming an electric capacitor which comprises:continuously rolling a flexible laminate including a metallic layer andan insulating layer upon a rotatable driven mandrel within an evacuatedenclosure to form a cylindrical body having a metallic outer surface;evaporating a dielectric material consisting essentially of amorphouszinc sulfide containing 0.01 to 10.0 weight percent of manganese andselectively directing the vapors thereof to a first portion of saidcylinder following the point at which the laminate first conforms to thecylindrical shape of said body to deposit thereupon a thin layer of saiddielectric, said insulating layer of the flexible laminate being atleast as thick as the dielectric layer so deposited; and simultaneouslyevaporating an easily volatizable metal and selectively directing thevapors thereof to a second portion of said cylinder angularly displacedin the direction of rotation of the body from said first portion todeposit a thin metallic layer upon said dielectric layer.

3. The method of forming an electric capacitor which comprises:continuously rolling a thin metal foil having on one side thereof anonconducting electrically insulating film upon a rotatable drivenmandrel within an evacuated enclosure to form a cylindrical body havinga metallic outer surface; evaporating a dielectric material consistingessentially of amorphous zinc sulfide containing 0.01 to 10.0 weightpercent of manganese and selectively directing the vapors thereof to afirst portion of said cylinder following the point at which the metalfoil firs t eonfo-rms to the cylindrical shape qf said body; said firstpontiqp t deposijg aythin metallicv lgyer upqp;

16 deposit athifi layer of d ieleqtr ic ther eupqn, said dieletrie lgyelnoneepducting electrically ins ul-agc ing on ne side of Saidmetal foilbeing at least; as thick as the d ielecgrie Re/feflfences cited" mthgfile cflthislpatqlt ye p si d'; s mul n o s y po i a a y 5 UNl EQ AI S,P N S volatizable metal and selectively directing the vapors 1,7;84g611.PolanyLet al. Dec. 9; 1930 thereof to, a second portion of said cylinderangularly 2,732,312 Young Jan; 24, 1956 displaced in the direction ofrotation of the body from 2,740,926 Ward Apr. 3, I956

1. THE METHOD OF FORMING AN ELECTRIC CAPACITOR WHICH COMPRISES:CONTINUOUSLY ROLLING A FLEXIBLE LAMINATE COMPRISING A METALLIX LAYER ANDAN INSULATING LAYER UPON A ROTATABLE DRIVEN MANDREL WITHIN AN EVACUATEDENCLOSURE TO FORM A CYLINDRICAL BODY HAVING A METALLIC OUTER SURFACE:EVAPORATING A DIELECTRIC MATERIAL CONSISTING ESSENTIALLY OF AMORPHOUSZINC SULFIDE CONTAINING 0.01 TO 10.0 WEIGHT PERCENT OF MAGNESE ANDSELECTIVELY DIRECTING THE VAPORS THEREOF TO A FIRST PORTION OF SAIDCYLINDER FOLLOWING THE POINTS AT WHICH THE LAMINATE FIRST CONFORMS TOTHE CYLINDRICAL SHAPE OF THE BODY TO DEPOSIT THEREUPON A THIN LAYER OFTHE DIELECTRIC; AND SMULTANEOUSLY EVAPORATING AN EASILY VOLATIZABLEMETAL AND SELECTIVELY DIRECTING THE VAPORS THEREOF TO A SECOND PORTIONOF SAID CYLINDER ANGULARLY DISPLACED IN THE DIRECTION OF ROTATION OF THEBODY FROM SAID FIRST PORTION TO DEPOSIT A THIN METALLIC LAYER UPON SAIDDIELECTRIC LAYER.